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from the NO + CH 3 O 2 reaction on the global distributions of alkyl nitrates, nitrogen oxides and
tropospheric ozone: a global modelling study
J.E. Williams, G Le Bras, Alexandre Kukui, H Ziereis, C. A. M.
Brenninkmeijer
To cite this version:
J.E. Williams, G Le Bras, Alexandre Kukui, H Ziereis, C. A. M. Brenninkmeijer. The impact of the chemical production of methyl nitrate from the NO + CH 3 O 2 reaction on the global distributions of alkyl nitrates, nitrogen oxides and tropospheric ozone: a global modelling study. Atmospheric Chemistry and Physics, European Geosciences Union, 2014, 14, pp.2363-2382. �10.5194/acp-14-2363- 2014�. �insu-01142123�
www.atmos-chem-phys.net/14/2363/2014/
doi:10.5194/acp-14-2363-2014
© Author(s) 2014. CC Attribution 3.0 License.
Atmospheric Chemistry and Physics
The impact of the chemical production of methyl nitrate from the NO + CH3O2 reaction on the global distributions of alkyl nitrates, nitrogen oxides and tropospheric ozone: a global modelling study
J. E. Williams1, G. Le Bras2, A. Kukui3, H. Ziereis4, and C. A. M. Brenninkmeijer5
1Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
2Institut de Combustion, Aérothermique, Réactivité et Environnement, CNRS, Orléans, France
3Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, CNRS, Orléans, France
4DLR, Oberpfaffenhofen, Germany
5Max Planck Institute for Chemistry, Atmospheric Chemistry, Mainz, Germany Correspondence to: J. E. Williams (williams@knmi.nl)
Received: 28 April 2013 – Published in Atmos. Chem. Phys. Discuss.: 2 August 2013 Revised: 9 December 2013 – Accepted: 9 January 2014 – Published: 7 March 2014
Abstract. The formation, abundance and distribution of or- ganic nitrates are relevant for determining the production efficiency and resident mixing ratios of tropospheric ozone (O3) on both regional and global scales. Here we investigate the effect of applying the recently measured direct chem- ical production of methyl nitrate (CH3ONO2) during NOx
recycling involving the methyl-peroxy radical on the global tropospheric distribution of CH3ONO2and the perturbations introduced towards tropospheric NOxand O3using the TM5 global chemistry transport model. By comparisons against numerous observations, we show that the global surface dis- tribution of CH3ONO2 can be largely explained by intro- ducing the chemical production mechanism using a branch- ing ratio of 0.3 %, when assuming a direct oceanic emission source of∼0.15 Tg N yr−1. On a global scale, the chemi- cal production of CH3ONO2converts 1 Tg N yr−1from ni- trogen oxide for this branching ratio. The resident mixing ratios of CH3ONO2 are found to be highly sensitive to the dry deposition velocity that is prescribed, where more than 50 % of the direct oceanic emission is lost near the source regions, thereby mitigating the subsequent effects due to long-range and convective transport out of the source re- gion. For the higher alkyl nitrates (RONO2) we find im- provements in the simulated distribution near the surface in the tropics (10◦S–10◦N) when introducing direct oceanic emissions equal to∼0.17 Tg N yr−1 . In terms of the ver- tical profile of CH3ONO2, there are persistent overestima-
tions in the free troposphere and underestimations in the up- per troposphere across a wide range of latitudes and longi- tudes when compared against data from measurement cam- paigns. This suggests either a missing transport pathway or source/sink term, although measurements show significant variability in resident mixing ratios at high altitudes at global scale. For the vertical profile of RONO2, TM5 performs bet- ter at tropical latitudes than at mid-latitudes, with similar fea- tures in the comparisons to those for CH3ONO2. Compar- isons of CH3ONO2 with a wide range of surface measure- ments shows that further constraints are necessary regard- ing the variability in the deposition terms for different land surfaces in order to improve on the comparisons presented here. For total reactive nitrogen (NOy) ∼20 % originates from alkyl nitrates in the tropics and subtropics, where the introduction of both direct oceanic emissions and the chemi- cal formation mechanism of CH3ONO2only makes a∼5 % contribution to the total alkyl nitrate content in the upper tro- posphere when compared with aircraft observations. We find that the increases in tropospheric O3that occur due oxida- tion of CH3ONO2originating from direct oceanic emission is negated when accounting for the chemical formation of CH3ONO2, meaning that the impact of such oceanic emis- sions on atmospheric lifetimes becomes marginal when a branching ratio of 0.3 % is adopted.
1 Introduction
The chemical production of tropospheric ozone (O3) is crit- ically dependent on the recycling efficiency of NO to NO2 involving peroxy radicals (Atkinson, 2000). The most abun- dant types of peroxy-radicals (RO2) in the troposphere are the hydro peroxy (HO2) and methyl peroxy (CH3O2) rad- icals, which are predominantly formed during the photoly- sis of chemical precursors such as formaldehyde (HCHO), and via oxidation of carbon monoxide (CO) and methane (CH4) during recycling of OH radicals. Once formed in Re- actions (R1)–(R3), NO2is rapidly photolysed, producing tro- pospheric ozone (O3) by Reactions (R4) and (R5):
NO+HO2→NO2+OH, (R1)
NO+CH3O2(+O2)→NO2+HCHO+HO2, (R2)
NO+RO2→NO2+RO, (R3)
NO2+hv→NO+O3P, (R4)
O3P+O2+M→O3+M. (R5)
Loss of NO may also occur via the titration of O3 (Reac- tion (6)), which moderates O3mixing ratios in high NOxen- vironments:
NO+O3→NO2+O2 (R6)
On a global scale, Reaction (1) is the dominant NOx recy- cling mechanism involving RO2; it accounts for∼65 % of total regeneration of NO2, while Reactions (2) and (3) ac- counting for∼25 % and∼10 %, respectively (see Sect. 5).
However, Reaction (6) is the major NO-to-NO2 recycling mechanism, where there is no net contribution towards the production of O3 under steady state NOx-O3 conditions, meaning that it acts as a moderating step.
The chain length of the free-radical reaction cycle de- scribed above is determined by the efficiency of the chain termination steps in high NOxenvironments, mainly Reac- tions (R7)–(R9) below. These lead to the formation of oxi- dised nitrogen reservoirs: nitric acid (HNO3), peroxy-acetyl nitrate (PAN) and organic nitrates (RONO2), respectively.
NO2+OH+M→HNO3+M (R7)
NO2+CH3(O)O2+M→PAN+M (R8)
NO+RO2+M→RONO2+M (R9)
Although the individual steps of this reaction cycle have been studied extensively over the last decades, there is still some uncertainty related to the possibility of long lived ni- trogen reservoirs being formed directly during Reactions (1) and (2). For instance, recent laboratory measurements us- ing chemical ionisation mass spectroscopy have revealed
the direct formation of HNO3from Reaction (1), where the branching ratio depends on pressure, temperature and rela- tive humidity (Butkovskaya et al., 2005, 2007). This has led to a number of different global modelling studies associated with studying the impact of this branching ratio on tropo- spheric composition (Cariolle et al., 2009; Sovde et al., 2011;
Gottschaldt et al., 2013; Boxe et al., 2012). The subsequent reduction in the recycling efficiency of NO reduces the pro- duction of tropospheric O3, which lowers oxidative capacity.
However, some ambiguity still exists as to the catalytic ef- fects of water vapour on the branching ratio (Butkovskaya et al., 2009) meaning that the direct formation of HNO3is typically not included in chemical mechanisms employed in large-scale chemistry transport models (CTMs) nor cur- rently included in the recommendations (e.g. Sander et al., 2011) due to the lack of an independent measurement for this branching ratio.
Using the same technique, Butkovskaya et al. (2012) recently detected the direct formation of methyl nitrate (CH3ONO2) from Reaction (2). The branching ratio (Reac- tion 10) inferred for tropospheric conditions is 1.0±0.7 %, and it has a weak temperature and pressure dependence:
NO+CH3O2+M→CH3ONO2+M. (R10) Results from a recent conceptual modelling study by Farmer et al. (2011) have suggested that the formation of RONO2
during the conversion of NO to NO2 by RO2 under urban conditions has the potential to significantly reduce the photo- chemical production of O3. Moreover, RONO2has also been found to be important for the lifetime of NOx whenever the total NOx mixing ratio is lower than 500 ppt (Browne and Cohen, 2012). Therefore, given the importance of Reac- tion (2) in the remote tropical troposphere in terms of NOx recycling, where the largest amount of CH4is oxidised (Fiore et al., 2006), Reaction (10) could have implications for the global tropospheric O3burden. The formation of CH3ONO2 from the sequestration of NO2by the CH3O radical has been invoked as another possible source, but it was considered in- significant on a global scale because of the high NO2mixing ratios which are necessary (Flocke et al., 1998a). The for- mation of CH3ONO2during the decomposition of PAN was also suggested, although this mechanism was later shown to be rather inefficient (Orlando et al., 1992) and therefore is not included in this study.
Alkyl nitrates contribute significantly to the total reac- tive nitrogen (NOy) budget of the troposphere (e.g. Buhr et al., 1990) but measuring them remains difficult. Nonethe- less they have been measured under a wide range of at- mospheric conditions (e.g. Flocke et al., 1998a, b; Blake et al., 1999; Swanson et al., 2003; Jones et al., 2011) typ- ically by analysing flask measurements, although in some instances techniques such as gas chromatography with elec- tron capture (Roberts et al., 1998), gas chromatography/mass spectroscopy (GC–MS; e.g. Dahl et al., 2005) and ther- mal dissociation/laser-induced fluorescence (TD–LIF; e.g.
Table 1. (a) Mixing ratios of CH3ONO2measured at the surface over the last few decades. (b) The range of CH3ONO2mixing ratios measured in the FT (between 2 and 10 km) during research flights over the last few decades from various measurement campaigns.
Region Latitude Lat./Long. Mixing ratios (ppt) Reference (a)
NH 73◦N 38◦W ∼4 Swanson et al. (2003)
44◦N 66◦W ∼2 Roberts et al. (1998)
Tropics 20◦N 156◦W ∼4 Walega et al. (1992)
0–17◦N 150◦E–150◦W 2–50 Dahl et al. (2005) 18◦S–30◦N 50◦W–5◦E 4–40 Chuck et al. (2002)
16–24◦S 133–145◦E ∼5 Simpson et al. (2002)
SH 70◦S 8◦W 8–10 Weller et al. (2002)
75.4◦S 26.4◦W 2–14 Jones et al. (2011)
88–90◦S ALL ∼3–11 Swanson et al. (2004)
88-90◦S ALL ∼4–13 Beyersdorf et al. (2010)
(b)
NH 40–85◦N 60–110◦W ∼2–4 Blake et al. (2003b)
30–40◦N 20–40◦W ∼2–3 Reeves et al. (2007)
Tropics/mid-lat. 2◦S–60◦N 115–155◦W ∼3–4 Flocke et al. (1998a) 40◦S–40◦N 150◦E–140◦W 1–27 Blake et al. (2003a) Global 60◦S–80◦N 160◦E–150◦W ∼3–40 Blake et al. (1999)
Farmer and Cohen, 2008) have been used at the measure- ment location. In Tables 1a and b, we provide an overview of mixing ratios of CH3ONO2 which have been measured at both the surface and in the free troposphere (FT, between 2–10 km), respectively, where values are grouped in latitu- dinal zones. Although a large fraction of alkyl nitrates are formed in the gas phase via Reaction (9), other direct sources are thought to exist from direct oceanic emissions (Atlas et al., 1993; Chuck et al., 2002; Dahl et al., 2005) and, to a lesser extent, biomass and savannah burning (Simpson et al., 2002). Measurements of alkyl nitrates made over the trop- ical ocean, along with concentration gradients of dissolved RONO2 measured in seawater surface, infer aqueous phase production until supersaturation occurs, followed by a re- lease into the Marine Boundary Layer (MBL), with a de- pendency on the dissolved nitrite concentrations (Dahl and Saltzman, 2008). Such oceanic emissions act as a direct source of additional nitrogen reservoirs into low NOxenvi- ronments without the need of long-range transport. This bio- genic source is likely to exhibit a latitudinal gradient related to the temperature of the surface waters and biological activ- ity (Chuck et al., 2002; Dahl et al., 2007). For instance, mea- surements made at coastal sites in the US have found that direct emission from the ocean waters only makes a minor contribution to resident mixing ratios of CH3ONO2near the east coast (Russo et al., 2010). Measurements on the Antarc- tic around 70–75◦S have revealed surprisingly high resident mixing ratios of between 2–14 ppt CH3ONO2, indicating a possible link to NOxemissions from snow (e.g. Weller et al., 2002; Wang et al., 2007). Compared to other alkyl nitrates, CH3ONO2is the most abundant at these southerly latitudes
(Jones et al., 2011). Similar measurements at the South Pole show a maximum of C1–C3 alkyl nitrates during austral win- tertime (Beyersdorf et al., 2010). Here the authors propose the source not to be direct emissions from the snowpack, but rather transport from oceanic regions. However, there is little vertical structure in the regional profiles shown, meaning that any oceanic source would have to be rather weak. Therefore, the actual source of CH3ONO2at these southerly latitudes is still the subject of some debate.
To investigate the global impacts on atmospheric compo- sition, Neu et al. (2008) used a CTM to show that biogenic alkyl nitrates exert an impact on the tropospheric O3 and NOx budgets in the tropics. In their study they adopted a priori emission estimates based on the measurements pre- sented in Blake et al. (2003a) for both CH3ONO2and ethyl nitrate (C2H5ONO2). However, the chemical formation of CH3ONO2via Reaction (10) was not considered. Including Reaction (10) could go some way to accounting for the ob- served resident mixing ratios in the remote marine tropo- sphere, on Antarctica and in the FT (Blake et al., 2003a;
Walega et al., 1992; Flocke et al., 1998a). This would re- sult in a lower emission flux estimate being needed to ex- plain ambient air measurements. Once present, alkyl nitrates are removed by either deposition processes (e.g. Russo et al., 2010), direct photolytic dissociation, or oxidation by OH (Talukdar et al., 1997a, b) to release NO2. Thus the potential reduction in the production efficiency of tropospheric O3by Reaction (2) due to Reaction (10) has the potential to neu- tralise the effects of increased oceanic nitrogen emissions.
In this paper we investigate the global impact that the pro- posed chemical formation of CH3ONO2has on composition
of the global troposphere by using the tropospheric version of the global 3-D Chemistry Transport Model (CTM) TM5.
In Sect. 2 we provide a description of the version of TM5 used for the study, the emission estimates adopted for precur- sor trace gases, and the updates made compared to previous model versions and details of the inclusion of CH3ONO2. In Sect. 3 we examine the distribution of both CH3ONO2 and higher alkyl nitrates in TM5 after including both the direct oceanic emission sources and the chemical production from Reaction (10). We also examine how the introduction of extra nitrogen into the troposphere alters the reactive nitrogen bud- get. In Sect. 4 we compare the distribution of alkyl nitrates at high latitudes and in the tropics against those measured during a number of intensive campaigns, and we identify the sensitivity of the vertical and regional distribution towards assumptions in both emissions and chemical formation. In Sect. 5 we show the effects on tropospheric NOx, O3 and OH and discuss implications for the atmospheric lifetimes of greenhouse gases. Finally, in Sect. 6, we present our conclu- sions.
2 Model description 2.1 The TM5 model
In this study we use the 3-D global CTM TM5 (Huijnen et al., 2010; Williams et al., 2013) which employs the modi- fied CB05 chemical scheme. This mechanism has recently been comprehensively described and validated in Williams et al. (2013) so for brevity we do not provide many details here. We perform simulations at a horizontal resolution of 3◦×2◦ and use 34 vertical levels from the surface up to
∼0.5 hPa. TM5 is driven using meteorological fields taken from the ERA-interim re-analysis (Dee et al., 2011), using an update frequency of 3 h. The details of which parameters from the meteorological fields are used in TM5 are given in Huijnen et al. (2010). The chosen simulation year is 2008 and all simulations use 2007 as a one-year spin-up period to reach chemical equilibrium.
Some modifications which improve TM5 compared to pre- vious versions have also been incorporated (e.g. Williams et al., 2013) as described here. As this version of TM5 only con- tains tropospheric chemistry, observational constraints are applied in the stratosphere for species such as CH4and O3
(Huijnen et al., 2010). We also adopt new boundary condi- tions for HNO3 based on the HNO3/ O3 ratios derived us- ing latitudinal climatologies assembled from measurements by the ODIN instrument between 2001 and 2009 (Jégou et al., 2008; Urban et al., 2009), where instantaneous forcing is applied at 10 hPa. This provides a more realistic seasonal evolution of HNO3in the stratosphere when compared to that simulated when using the Upper Atmosphere Research Satel- lite (UARS) climatology in previous versions (Huijnen et al., 2010; Williams et al., 2012), especially in the polar regions.
Previous studies have shown that the global dry deposition flux of carbon monoxide (CO) in TM5 was rather high, ac- counting for the loss of 160–180 Tg CO yr−1(e.g. Williams et al., 2012). In this study the loss of CO at the surface by enzymatic processes in the soil (Yonemura et al., 2000) is parameterised with respect to the soil moisture content ac- cording to Sanderson et al. (2003). The resistive fluxes are then passed into the dry deposition scheme of Ganzeveld and Lelieveld (1995). This reduces the global dry deposition flux by around∼80 Tg CO yr−1. Further updates associated with dry deposition include increasing the surface resistance for CO over oceans and of O3 over snow and ice. Finally we improved the carbon balance in the CH3O2self-termination reaction by adopting the CB05 reaction nomenclature given in Yarwood et al. (2005).
For this study we have included CH3ONO2explicitly as an additional transported tracer species. The atmospheric life- time of CH3ONO2is estimated to range from around a week to a month depending on atmospheric conditions (Talukdar et al., 1997a, b), meaning that it can be transported out of the Boundary Layer (BL; the first few kilometres near the surface) into the FT and beyond. This is compared to atmo- spheric lifetimes of between several days to weeks for the C2–C5 RONO2 (Clemitshaw et al., 1997). Both the pho- tolytic destruction and oxidation of CH3ONO2 by OH are included using the chemical reaction data taken from Taluk- dar et al. (1997a, b) forming the products NO2and HCHO.
We also include a dry deposition term equal to that used for the C2–C4 RONO2 for the majority of the simulations. A small wet deposition term is also included, using the values of Kames and Schurath (1992).
2.2 Definition of the emission scenarios and sensitivity studies
The emission estimates used here are described in Williams et al. (2013), and are thus only briefly summarised here. For anthropogenic emissions we use a hybrid complied using the RETRO inventory (Schultz et al., 2007; http://retro.enes.org) and the REAS (Regional Emission Inventory in Asia) inven- tory for the Asian region (Ohara et al., 2007) between 60–
150◦E and 10◦S–50◦N. The exception is CH4, which we take from EDGARv4.2 (http://edgar.jrc.ec.europa.eu/). For biomass-burning emissions, we use the GFEDv3 monthly emission inventory (van der Werf et al., 2010). For biogenic emissions we use the estimates provided by MEGANv2 (Guenther et al., 2006) except for CH4, which we take from the LPJ-WHyMe (LPJ-Wetland Hydrology and Methane) in- ventory (Spahni et al., 2011). For lightning NOxwe use the parameterisation of Meijer et al. (2001). The CH4distribu- tion in the boundary layer is constrained by nudging to a sea- sonally dependant latitudinal gradient up to 500 hPa using the method given in Williams et al. (2013).
In total, six sensitivity studies are defined and listed in Table 2, in addition to the BASE simulation which does
Table 2. An overview of the sensitivity studies performed with TM5 to investigate the impact of the chemical production of CH3ONO2on global tropospheric composition.
Model simulation
Direct oceanic emissions
Branching ratio Comments
BASE NO N/A Default modified CB05 chemistry with-
out CH3ONO2 and no oceanic emis- sions
EMISS YES N/A Adopting the emission estimates for
CH3ONO2from Neu et al. (2008) be- tween 10◦S and 10◦N
DEMISS YES N/A Adopting an emission twice that of Neu
et al. (2008) between 10◦S–10◦N
LOWBR NO 0.3 % Lower uncertainty range from
Butkovskaya et al. (2012)
HIGHBR NO 1.0 % Branching ratio measured by
Butkovskaya et al. (2012)
FULL YES 0.3 % Combined emissions (0.5 of EMISS)
and direct production (as in LOWBR)
DDEP YES 0.3 % As FULL, except the dry deposition
flux is increased by 100 %
FLIGHT YES 0.025 % As DEMISS, except adopting the
branching ratio suggested in Flocke et al. (2003a) derived from measurements
not explicitly include CH3ONO2. These are, namely, (1) EMISS, which applies a direct a priori oceanic emission for CH3ONO2 of 26.7 mg m−2s−1 (∼0.29 Tg yr−1) as taken from Neu et al. (2008) and applied between 10◦S-10◦N; (2) DEMISS, which applies a direct a priori oceanic emission flux for CH3ONO2twice that of EMISS to represent an up- per limit; (3) LOWBR, with which we adopt a low branch- ing ratio of 0.3 % for the chemical formation of CH3ONO2 (the lower limit of the experimental data in Butkovskaya et al., 2012); (4) HIGHBR with a branching ratio of 1.0 %; (5) FULL, with which we reduce the direct emission by 50 % compared to EMISS and apply the low branching ratio of 0.3 %; (6) DDEP, applied in the same manner as FULL, ex- cept we double the dry deposition flux of CH3ONO2; and (6) FLIGHT, applied in the same manner as DEMISS, ex- cept that we use a branching ratio of 0.025 % which is the mean value suggested by Flocke et al. (1998a) derived from measurements made in the upper troposphere (UT; above 10 km until the tropopause). In the EMISS, FULL, DDEP and FLIGHT simulations we also include the direct emis- sions of RONO2 from the ocean, adopting an estimate of 15.62 mg m−2s−1(∼0.17 Tg N yr−1), which is twice the a priori estimate calculated for C2H5ONO2in Neu et al. (2008) so as to represent the release of other alkyl nitrates such as e.g. isopropyl nitrate as measured over the tropical ocean (Dahl et al., 2005). Direct emissions of alkyl nitrates from
the oceans are applied out away from coastlines by using the land–sea mask threshold of 15 % in each grid cell to exclude large freshwater lakes and landlocked bodies of water in the tropics. In our study – unlike in Neu et al. (2008), which was motivated by the measurements of Blake et al. (2003a) – no oceanic emissions are applied in the SH. The emission of alkyl nitrates from the ocean has been linked to both the con- centration of chlorophyll and sea temperature (Chuck et al., 2002; Dahl and Saltzman, 2008). A strong seasonality exists in the chlorophyll concentrations in the SH (Myriokefalitakis et al., 2010) resulting in maximal concentrations occurring during the boreal wintertime; therefore any resulting emis- sion flux is likely to exhibit a similar seasonality. The small emission of CH3ONO2measured from biomass and savan- nah fires (∼18 Gg yr−1, Simpson et al., 2002) is an order of magnitude lower than the ocean emission flux and thus is not included.
2.3 Observational data sets
To investigate to what degree TM5 is able to capture a re- alistic global distribution of alkyl nitrates on global scale we compare model output co-located with independent measure- ments made at selected stations and by research aircraft. Rel- evant details on these sites and campaigns are given in the following.
The Halley Bay research station at coastal Antarctica (75.3◦S, 26.4◦W) is situated 32 m above sea level and lo- cated on the Brunt Ice shelf. It is surrounded on three sides by the Weddell Sea and can be near ice-free water during the austral summer months. For our study we use the measurements presented in Jones et al. (2011) taken as part of the Chemistry of the Antarctic Boundary Layer and the Interface with Snow (CHABLIS) campaign which took place from February 2004 to January 2005. Measure- ments were performed on samples stored in flasks which were transported and analysed in the United Kingdom by gas chromatography–mass spectroscopy.
Neumayer, in Antarctica (70.0◦S, 8.2◦W), is a well- established measurement station. It is located on an ice shelf
∼10 km away from the northeastern Weddell Sea, and is considered to be representative of a location with pristine air transported either over the snow pack on Antarctica or from over the surrounding ocean and sea ice. Here we use the measurement values published by Weller et al. (2002), who analysed air from flasks by gas chromatography to determine mixing ratios of alkyl nitrates.
The Mauna Loa Observatory is located∼3.4 km above sea level in Hawaii (19.5◦N, 155.6◦W), which is situated in the remote northern Pacific Ocean. It has a long-standing his- tory of measuring atmospheric composition, and we use val- ues observed during the Mauna Loa Observatory Photochem- istry Experiment (MLOPEX, Ridley and Robinson, 1992).
These measurements are representative of FT air and there- fore should provide information regarding background mix- ing ratios. Again, the measured values published in Walega et al. (1992) are derived using gas chromatography with a detection limit of∼0.3 ppt for RONO2.
For the extratropical NH we use the values provided in Roberts et al. (1998) measured at Chebogue Point, Nova Sco- tia (43.8◦N, 66.1◦W) as part of the North Atlantic Regional Experiment (NARE; Fehsenfeld et al., 1996) during August and September 1993. The measurement site is located at the tip of a peninsula about 10 km south of Yarmouth, Nova Sco- tia. The location was chosen such that strong anthropogenic NOxsources towards the south did not impinge significantly on the measurements, although there was some local ship traffic (Fehsenfeld et al., 1996). Therefore, these measure- ments are thought to be representative of the remote condi- tions affected by oceanic air masses.
To assess the vertical distribution of CH3ONO2 and RONO2in the TM5 model at tropical latitudes, we use data obtained during a number of different flights conducted as part of the second Pacific Exploratory Mission (PEM-Tropics B; Raper et al., 2001) between March and April 1999 in the region 76◦E–148◦W and 36◦S–38◦N. Measurement data from both the DC-8 and P-3B aircraft have been used, which use different instrumentation for measuring the mixing ra- tios. For the comparison we interpolate three-hourly model data to the altitude and location at which the measurements were taken. The measurement data are averaged approxi-
mately every minute to avoid fluctuations in mixing ratios due to small-scale variability. These measurements have been presented in Blake et al. (2003a) and provide valuable infor- mation regarding the tropospheric distribution of these im- portant NOxreservoirs.
For the high northern latitudes we use measurements made during the Arctic Research of the Composition of the Tro- posphere from Aircraft and Satellites (ARCTAS) mission, which took place during two separate phases during April and June/July 2008 (Jacob et al., 2010). Only data from the DC-8 aircraft are used, which was based at Fairbanks (65◦N, 148◦W), Alaska during April and Cold Lake (54◦N, 110◦W), Alberta, Canada during June/July. As well as cap- turing some seasonality for the region, these measurements cover a large range of northern latitudes between 50–70◦N across a wide longitudinal range, where the details of exact locations are given in Jacob et al. (2010). For our compar- isons we use the same averaging technique to that utilised for the PEM-tropics B data, where the measurement data is again averaged approximately every minute. To our knowl- edge this is the first time that this alkyl nitrates data has been used in a global model study.
Finally, we use chemical NOy measurements from the second phase of the Civil Aircraft for Regular Investiga- tion of the Atmosphere Based on an Instrument Container (CARIBIC) project, which started in 2005 and continues to run (Brenninkmeijer et al., 2007). In this instance NOyis de- fined as the cumulative sum of all reactive nitrogen species, namely NO, NO2, PAN, HNO3, HNO4, N2O5and all alkyl nitrates. All trace constituents are reduced to NO during the measurement, meaning that no values are available for individual reservoir species. We use in situ measurements made from April until December 2008 made during flights from Frankfurt, Germany (50.0◦N, 8.5◦E) to Chennai, India (13.0◦N, 80.2◦E). The flights to and from Chennai take sim- ilar routes to those shown in Schuck et al. (2010), providing a monthly snapshot of the distribution of NOyat the cruise altitude of 10–12 km throughout a large fraction of the year.
The uncertainty associated with the CARIBIC NOymeasure- ments is ∼8 % for mixing ratios around 450 ppt. We only use measurements which are representative of the upper tro- posphere (UT), which restricts us to the latitude range 15–
30◦N.
3 The annual mean distribution of CH3ONO2and RONO2
Figure 1 shows both the annual zonal and horizontal mean distributions of CH3ONO2for EMISS, along with the cor- responding absolute differences in mixing ratios relative to FULL given in ppt, thus differentiating the impact of Reac- tion (10). For the annual horizontal mean distribution, mix- ing ratios are averaged between the surface and 800 hPa so as to give a representative value of the first few kilometres of
Fig. 1. The annual mean distribution in CH3ONO2 during 2008.
Both the zonal and horizontal means are shown (top and bottom, respectively) for EMISS. The right panels show the absolute differ- ences for FULL-EMISS given in ppt.
the troposphere. The total cumulative emission of CH3ONO2
in EMISS is equivalent to∼0.3 Tg N yr−1 and results in a global tropospheric burden of 0.015 Tg N. The highest mix- ing ratios of between 40–60 ppt occur in the tropical MBL di- rectly above the emission sources, where CH3ONO2is lifted out of the MBL reaching the UT. Long-range transport to- wards higher latitudes then occurs by the large-scale atmo- spheric circulation. Unlike the vertical distribution shown in Neu et al. (2008), which exhibits maximal mixing ratios both in the BL and higher up in the UT, we do not get any in- creases in mixing ratios above 10 km in EMISS, but rather a continuous decrease as determined by the increasing ef- ficiency of photolytic destruction with altitude and that the only source in EMISS is emission at the surface. We also do not have high mixing ratios in the SH as we only in- troduce oceanic emissions in the tropics, since the location of oceanic emissions is constrained by observations made by ships (e.g. Dahl et al., 2005). One complication is that there appears to be a large variability in the vertical profile of CH3ONO2 at similar latitudes from the available measure- ments when comparing campaigns. For instance, the mea- surements of Flocke et al. (1998a) indicate that the mixing ratio of CH3ONO2should be between 1–3 ppt around 11km at tropical latitudes, which almost matches the zonal mean values shown for EMISS in Fig. 1. However, the profiles shown in Blake et al. (2003a) show higher mixing ratios in the tropical UT towards the Pacific (of between 10–15 ppt).
This implies that there is a significant longitudinal variability
in emission sources as determined by differences in biogenic activity. The long-range transport of CH3ONO2 out of the tropics below 800 hPa at higher latitudes is shown to be neg- ligible due to loss by deposition at the surface. The global tropospheric lifetime of CH3ONO2in EMISS is∼20 days, well within the estimates of between 7 days to a month as given in the literature (Butkovskaya et al., 2012). Chemi- cal destruction predominantly occurs in the tropics, where dry deposition accounts for ∼50 % of the total sink term.
However, in these simulations we did not employ a specific deposition flux for CH3ONO2, but used that of RONO2 – which is assumed to be equal to the deposition flux of PAN – as a proxy (Huijnen et al., 2010). Comparing these solu- bility parameters to those given in Sander (1999) shows that this estimate is potentially too high by∼25 %. However for CH3ONO2, Russo et al. (2010) needed to increase the depo- sition velocity to values similar to that of O3in order to fit measured diurnal profiles. This motivated us to perform the DDEP simulation using enhanced deposition rates, which is discussed later on.
When introducing Reaction (10), absolute differences of between 20–60 ppt occur in the mean CH3ONO2mixing ra- tios. The largest increases are situated over continental re- gions in the NH around the subtropics and tropics in the FT, above where most surface NOx is emitted. It should be noted that these increases occur in spite of the 50 % reduc- tion in the direct oceanic emission term for CH3ONO2 in FULL, to account for the introduction of a mixed source (see Table 2). This reduces the direct emission of CH3ONO2 to
∼0.15 Tg N yr−1 in FULL. Table 3 provides an annual tro- pospheric chemical budget for EMISS and FULL, along with that for other selected simulations. The troposphere is de- fined as the 150 ppb monthly mean contour in tropospheric O3, as described in Stevenson et al. (2006). Comparing terms given in Table 3 shows that the direct emission of CH3ONO2 applied in FULL is smaller than the total formed in Reac- tion (10) by approximately a factor of seven. The global tro- pospheric burden of CH3ONO2 in FULL increases signifi- cantly, to∼0.08 Tg N (∼more than six times that found in EMISS). This increases the resulting global CH3ONO2life- time to 25.3 days, this time with chemical destruction being the dominant loss term (∼55 %) due to a smaller fraction of CH3ONO2being near the surface.
When analysing the horizontal distribution of the abso- lute changes in the mixing ratios in Fig. 1 it follows that the highest differences are situated in the NH between 10–
30◦N over northern Africa, the Middle East and India. Part of this variability can be related to the regional differences in the deposition terms for CH3ONO2 as shown in Fig. 2.
Here the regional variability in annual deposition terms from FULL is given, where the integrated annual deposition varies by an order of magnitude depending on surface type and lo- cal mixing ratios. The highest annual deposition occurs over the tropical oceans near regions close to where direct oceanic emissions are prescribed. The lowest deposition occurs over
Table 3. The annual tropospheric chemical budget terms for CH3ONO2for 2008 given in Tg N yr−1for selected simulations.
EMISS DEMISS LOWBR FULL FLIGHT
Emission 0.286 0.573 0.000 0.142 0.573
NO+CH3O2 N/A N/A 0 .990 0.994 0.084
OH+CH3ONO2 0.011 0.023 0.047 0.053 0.027 CH3ONO2+hν 0.124 0.248 0.476 0.540 0.289
Dry deposition 0.139 0.279 0.429 0.500 0.317
Wet deposition 0.008 0.017 0.006 0.017 0.020
Fig. 2. The annually integrated global deposition flux of CH3ONO2 during 2008 from FULL. The values are given in 106mg N m−2yr−1.
arid regions such as the Sahara and Antarctica. Examining the chemical budget terms shows that approximately half of the CH3ONO2 released from tropical emissions is lost by local deposition. This rapid turnover highlights the impor- tance of placing constraints on deposition velocities by mea- surements, especially over the ocean, whose surface area in the tropics exceeds that of the continents. Such information would help the modelling community in being able to gauge the ability of atmospheric models to capture this important physical sink process. For the fraction of CH3ONO2 oxi- dised, photolytic destruction accounts for > 90 % of the total loss term. When comparing the chemical budget terms be- tween EMISS and FULL it can be seen that Reaction (10) significantly reduces the contribution of direct oceanic emis- sions of CH3ONO2to the global nitrogen budget. We discuss the implications for tropospheric O3 production and oxida- tive capacity later on in Sect. 5.
To highlight the variability which is introduced across sim- ulations with respect to the distribution of CH3ONO2, Fig. 3 shows the annual horizontal mean distribution of CH3ONO2 for DEMISS, HIGHBR, LOWBR and FLIGHT, respectively, again avergaed between the surface and 800 hPa. Table 3 provides the corresponding annual chemical budget terms for the majority of these simulations, allowing for the com-
Fig. 3. The annual mean near surface distribution of CH3ONO2 during 2008 for (a) DEMISS, (b) HIGHBR (c) LOWBR and (d) FLIGHT given in ppt.
parison of source and sink terms. Comparing the fractional loss due to deposition in Table 3 for EMISS and DEMISS shows that when restricting the source of CH3ONO2to sur- face emissions, the amount lost by deposition scales rather linearly with the prescribed oceanic emission flux. Thus, the impact of direct emissions of CH3ONO2 is much less than from other sources of NOxsuch as lightning or aircraft emis- sions, which occur in the FT due to the mitigation intro- duced by deposition. Figure 3 shows that the chemical pro- duction is most efficient where the chemical precursors have the highest mixing ratios, i.e. between 30◦S and 40◦N (not shown). Comparing the surface distribution of CH3ONO2 in HIGHBR and values presented in Table 1b from surface measurements shows that using a 1 % branching ratio over- estimates observed values by an order of magnitude, and for
Table 4. The annual tropospheric chemical budget terms for RONO2 for 2008 as calculated in the modified CB05 chemical mechanism as given in Tg N yr−1 for selected TM5 simulations.
The term for total nitrogen deposition is defined as the sum of dry and wet deposition of NO2, HNO3, RONO2and PAN.
BASE FULL
Emission N/A 0.168
NO+XO2N 11.205 11.197 NO3+C3H6 0.056 0.056 NO3+ISOP 2.317 2.311 NO3+TERP 1.010 1.008 OH+RONO2 1.289 1.313 RONO2+hν 4.489 4.581 Dry deposition 2.874 2.940 Wet deposition 6.225 6.343 Global burden 0.173 0.176 Total N deposition 48.390 48.541
this reason, this simulation is not discussed further here. For LOWBR, mixing ratios between 20–70 ppt occur between 30◦S–30◦N, where maximal mixing ratios occur over ter- rain with low deposition velocities (cf. Fig. 2). There is also an amplification of a few percent in the total chemical pro- duction term as a consequence of introducing oceanic emis- sions, although this is a rather weak feedback linked to en- hancements in oxidative capacity (and thus CH3O2).
Figure 4 shows the corresponding global distribution of RONO2 in BASE and Table 4 provides the corresponding global chemical budget terms for RONO2in both the BASE and FULL. The absolute differences between these simu- lations in mean mixing ratios for RONO2 are also shown.
Analysing the chemical budget for BASE reveals that in the modified CB05 mechanism, RONO2 is predominantly formed via the conversion of NO with the NOx operator species XO2N (Gery et al., 1989) and the oxidation of iso- prene with the nitrate radical during nighttime. Figure 4 shows that the highest mixing ratios occur in regions where both anthropogenic and biogenic emissions dominate (e.g.
eastern Asia and over Amazonia). Analysing the annual zonal mean shows that in the FT mixing ratios are higher in the NH. Introducing direct oceanic emissions increases the resident mixing ratios by∼100 % at the surface and between
∼5–20 % in the FT. When examining the absolute differ- ences in RONO2shown in Fig. 4, one can see that the largest occur in the tropics directly over the emission source regions.
As for CH3ONO2, the chemical production term dominates over the direct emission. The global tropospheric burden of RONO2in the FULL simulation is 0.18 Tg N, giving a global tropospheric lifetime of 4.2 days, shorter than the value given for mid-latitudes in the literature by about a week (Clemit- shaw et al., 1997). This is possibly due to 50 % being lost
Fig. 4. The annual global distribution in RONO2during 2008 given in ppb. Both the zonal and horizontal (< 800 hPa) means are shown (top and bottom, respectively) for the BASE simulation. The abso- lute differences when compared against the FULL simulation are also given, with differences being in the ppt range.
in the tropics and that adopting the deposition flux of PAN as a proxy results in too efficient loss by deposition, which exceeds that lost by chemical destruction. Finally, in Table 4 we also show the total summed N deposition terms for NO2, HNO3, PAN and RONO2. Here N deposition increases by∼0.15 Tg N yr−1 (also for EMISS, not shown); thus ul- timately the additional N introduced by direct emission is almost entirely lost via deposition.
4 Comparisons against surface and aircraft measurements
Over the last few decades there have been a number of mea- surement campaigns and initiatives on different scales that have observed CH3ONO2mixing ratios at pristine locations and throughout the FT providing some information regard- ing background mixing ratios found in the troposphere. In Tables 1a and b we have summarised the resident mean mix- ing ratios of CH3ONO2available in the literature from the last few decades, showing the vertical and latitudinal vari- ability which exists on a global scale. For our purpose we subsequently select some of the surface sites, aircraft mea- surements at high northern latitudes and the measurements of Blake et al. (2003a, b) in the tropics to validate the global distribution of CH3ONO2in TM5.
